1. Field of the Invention
The present invention relates to formation cleanup and more particularly to a technique to greatly improve the efficiency of matrix acidizing and formation completion cleanup in horizontal wellbores.
2. Related Prior Art
With continued emphasis on drilling and completion of horizontal wellbores into a variety of oil and gas-bearing formations, cleanup of the wellbore and surrounding area is becoming increasingly important. A horizontal wellbore may become severely damaged by drilling mud invasion, loss of completion fluids, which is typically a weighted gel brine, or kill pill residue. If this happens, many of the original purposes of the horizontal wellbore, which are improved productivity, better reservoir drainage, minimization of water coning, etc., are lost.
Several methods are available in the industry to help clean up damage to horizontal wellbores, such as acidizing with foam diversion, placement of stimulation fluids with coiled tubing, etc. What is needed, however, is a method to improve the efficiency of the horizontal wellbore cleanout in order to take full advantage of the improved productivity afforded by the horizontal well.
There are methods for providing access to oil in difficult formations, particularly carbonate formations, and for repairing damage done to horizontal hydrocarbon producing wells that may occur during its hydrocarbon producing life. Several patents and articles are listed below that are indicative of the state of the art in production enhancement in carbonate formations in horizontal wells.
U.S. Pat. No. 4,883,124, titled "Method of Enhancing Hydrocarbon Production in a Horizontal Wellbore in a Carbonate Formation", issued to Alfred R. Jennings, Jr., relates to a two step process to stimulate a horizontal wellbore drilled into a carbonate formation. Initially, the wellbore is filled with acid. Because vertical communication exists in the vicinity of the wellbore, the acid enters into the fissures and cracks from the wellbore. Thereafter, a non-reactive displacement fluid, having a density greater than the acid, is injected into the wellbore. This more dense displacement fluid selectively pushes the acid to greater depths into the formation so carbonate dissolution can take place which substantially increases the formation's permeability. Increased permeability enhances the production of hydrocarbonaceous fluids.
U.S. Pat. No. 4,951,751, titled "Diverting Technique to Stage Fracturing Treatments in Horizontal Wellbores", issued to Alfred R. Jennings, Jr., relates to a method for staging a fracturing treatment in a horizontal wellbore where solidified gel is used as a diverting medium. A desired section of the horizontal wellbore farthest removed from the angle of deviation from vertical of the wellbore is perforated. Through perforations contained in the horizontal section, the desired interval is fractured hydraulically. The gel is displaced with a "wiper plug" and the gel confined to the fractured interval and wellbore area adjacent the fractured interval. Here the gel forms a solid gel in the interval and a gel plug in the wellbore. Afterwards, another section of the horizontal well is perforated. Thereafter, a second desired interval is fractured. After completion of the fracturing process, the gel plug breaks and the "wiper plug" is pumped to the farthest end of the horizontal wellbore.
"Effect of Foams Used During Carbonate Acidizing" by M. G. Bernadiner, SPE, K. E. Thompson, SPE, and H. S. Fogler, SPE, U. of Michigan, published in SPE Production Engineering, November 1992, states that although acidization has been used successfully for many years to increase the productivity of petroleum wells in carbonate formation, demands on the performance and application of the acidizing process are increasing. This study investigated a method of in-situ foam generation that allows deeper wormhole penetration yet uses less acid than conventional methods. The dissolution patterns were imaged with neutron radiography, which provided an in depth understanding of the effects of foam and other critical parameters. Results show that foam is effective in promoting efficient stimulation, even at low acid injection rates.